docs: Discourage users from using fwcfg
docs: Discourage users from using fwcfg

Even though this was brought up in upstream discussion [1] it
missed my patches: users should prefer <oemStrings/> over fwcfg.
The reason is that fwcfg is considered somewhat internal to QEMU
and it has limited number of slots and neither of these applies
to <oemStrings/>.

While I'm at it, I'm fixing the example too (because it contains
incorrect element name) and clarifying sysfs/ exposure.

1: https://www.redhat.com/archives/libvir-list/2020-May/msg00957.html

Reported-by: Laszlo Ersek <7437296077edea1cb64451c122c4dac071f0cfbc@redhat.com>
Signed-off-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>
Reviewed-by: Laszlo Ersek <7437296077edea1cb64451c122c4dac071f0cfbc@redhat.com>

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